1. Field of the Invention
The present invention relates to a method of non-invasively measuring the cardiac output of a patient. Particularly, the present invention relates to a method of measuring cardiac output which accounts for the amount of intrapulmonary shunted blood. More particularly, the present invention relates to a method of non-invasively estimating intrapulmonary shunt and considering the intrapulmonary shunt with re-breathing pulmonary capillary blood flow measurements in measuring the cardiac output.
2. Background of Related Art
Cardiac output is one of various hemodynamic parameters that may be monitored in critically ill patients. Conventionally, cardiac output has been measured by direct, invasive techniques, such as by thermodilution using a Swan-Ganz catheter. Invasive measurement of cardiac output is undesirable because of the potential for harming the patient that is typically associated with the use of such a catheter.
Thus, non-invasive techniques for determining cardiac output have been developed. Cardiac output is the sum of blood flow through the lungs that participates in gas exchange, which is typically referred to as pulmonary capillary blood flow, and the blood flow that does not participate in gas exchange, which is typically referred to as intrapulmonary shunt flow or venous admixture.
The pulmonary capillary blood flow of a patient has been non-invasively determined by employing various respiratory, blood, and blood gas profile parameters in a derivation of the Fick equation (typically either the O.sub.2 Fick equation or the CO.sub.2 Fick equation), such as by the use of partial and total re-breathing techniques.
The carbon dioxide Fick equation, which may be employed to determine cardiac output, follows:
Q.sub.t =VCO.sub.2 /(CVCO.sub.2 -CaCO.sub.2),
where Q.sub.t is the cardiac output of the patient, VCO.sub.2 is the carbon dioxide elimination of the patient, CVCO.sub.2 is the carbon dioxide content of the venous blood of the patient, and CaCO.sub.2 is the carbon dioxide content of the arterial blood of the patient.
The carbon dioxide elimination of the patient may be non-invasively measured as the difference per breath between the volume of carbon dioxide inhaled during inspiration and the volume of carbon dioxide exhaled during expiration, and is typically calculated as the integral of the carbon dioxide signal times the rate of flow over an entire breath. The volume of carbon dioxide inhaled and exhaled may each be corrected for any deadspace or for any intrapulmonary shunt.
The partial pressure of end tidal carbon dioxide (PetCO.sub.2 or etCO.sub.2) is also measured in re-breathing processes. The partial pressure of end-tidal carbon dioxide, after correcting for any deadspace, is typically assumed to be approximately equal to the partial pressure of carbon dioxide in the alveoli (PACO.sub.2) of the patient or, if there is no intrapulmonary shunt, the partial pressure of carbon dioxide in the arterial blood of the patient (PaCO.sub.2). Conventionally employed Fick methods of determining cardiac output typically include a direct, invasive determination of CVCO.sub.2 by analyzing a sample of the patient's mixed venous blood. The re-breathing process is typically employed to either estimate the carbon dioxide content of mixed venous blood (in total re-breathing) or to obviate the need to know the carbon dioxide content of the mixed venous blood (by partial re-breathing) or determine the partial pressure of carbon dioxide in the patient's venous blood (PVCO.sub.2).
Re-breathing processes typically include the inhalation of a gas mixture which includes carbon dioxide. During re-breathing, the carbon dioxide elimination typically decreases. In total re-breathing, carbon dioxide elimination decreases to near zero. In partial re-breathing, carbon dioxide elimination does not cease. Thus, in partial re-breathing, the decrease in carbon dioxide elimination is not as large as that of total re-breathing.
Re-breathing can be conducted with a re-breathing circuit, which causes a patient to inhale a gas mixture that includes carbon dioxide. FIG. 1 schematically illustrates an exemplary re-breathing circuit 50 that includes a tubular airway 52 that communicates air flow to and from the lungs of a patient. Tubular airway 52 may be placed in communication with the trachea of the patient by known intubation processes, or by connection to a breathing mask positioned over the nose and/or mouth of the patient. A flow meter 72, such as a pneumotachometer, and a carbon dioxide sensor 74, which is typically referred to as a capnometer, are disposed between tubular airway 52 and a length of hose 60, and are exposed to any air that flows through re-breathing circuit 50. Both ends of another length of hose, which is referred to as deadspace 70, communicate with hose 60. The two ends of deadspace 70 are separated from one another by a two-way valve 68, which may be positioned to direct the flow of air through deadspace 70. Deadspace 70 may also include an expandable section 62. A Y-piece 58, disposed on hose 60 opposite flow meter 72 and carbon dioxide sensor 74, facilitates the connection of an inspiratory hose 54 and an expiratory hose 56 to re-breathing circuit 50 and the flow communication of the inspiratory hose 54 and expiratory hose 56 with hose 60. During inhalation, gas flows into inspiratory hose 54 from the atmosphere or a ventilator (not shown). During normal breathing, valve 68 is positioned to prevent inhaled and exhaled air from flowing through deadspace 70. During re-breathing, valve 68 is positioned to direct the flow of exhaled and inhaled gases through deadspace 70.
During total re-breathing, the partial pressure of end-tidal carbon dioxide is typically assumed to be equal to the partial pressure of carbon dioxide in the venous blood (PVCO.sub.2) of the patient, as well as to the partial pressure of carbon dioxide in the arterial blood (PaCO.sub.2) of the patient and to the partial pressure of carbon dioxide in the alveolar blood (PACO.sub.2) of the patient. The partial pressure of carbon dioxide in blood may be converted to the content of carbon dioxide in blood by means of a carbon dioxide dissociation curve.
In partial re-breathing, measurements during normal breathing and subsequent re-breathing are substituted into the carbon dioxide Fick equation. This results in a system of two equations and two unknowns (carbon dioxide content in the mixed venous blood and cardiac output), from which pulmonary capillary blood flow can be determined without knowing the carbon dioxide content of the mixed venous blood.
Known re-breathing techniques for non-invasively determining cardiac output are, however, somewhat undesirable since they typically measure pulmonary capillary blood flow and do not account for intrapulmonary shunt flow.
The failure of conventional non-invasive re-breathing techniques for determining cardiac output to account for intrapulmonary shunt was recognized, and techniques were developed to estimate the intrapulmonary shunt. Some intrapulmonary shunt flow (Q.sub.s) or shunt fraction (Q.sub.s /Q.sub.t) or venous admixture estimates employ values obtained from pulse oximetry (SpO.sub.2) and inspiratory oxygen fractions (FiO.sub.2). In B. Osterlund et al., A new method of using gas exchange measurements for the noninvasive determination of cardiac output: clinical experiences in adults following cardiac surgery, Acta Anaesthesiol. Scand. (1995) 39:727-732 ("Osterlund"), Osterlund notes that while pulse oximetry measurements provide accurate shunt estimates when FiO.sub.2 is close to 0.21 (approximately the fraction of oxygen in the air), when the fraction of inspired oxygen (FiO.sub.2) exceeds 0.5, as it typically does when a patient is artificially ventilated, the arterial oxygen tension of a patient should be measured directly (i.e., invasively). Moreover, as FIG. 2 illustrates, as the blood becomes about 95-100% saturated with oxygen, due to the steepness of the oxygen tension-saturation curve of FIG. 2, precise and accurate arterial blood oxygen saturation measurements (SaO.sub.2) are necessary to accurately determine the partial pressure of oxygen in a patient's arterial blood. Thus, since pulmonary capillary blood flow measurements are often taken while a patient's breathing is artificially ventilated, and since FiO.sub.2 is typically greater than about 0.5, the technique disclosed in Osterlund often undesirably requires invasive measurement of SaO.sub.2.
Accordingly, there is a need for a method of non-invasively and accurately estimating intrapulmonary shunt, as well as a method of accounting for the estimated intrapulmonary shunt in determining the cardiac output of a patient by re-breathing techniques.